Smaller than a breadbox
Wired has an article about small form factor PCs. I have to say that I'm a fan. The most recent desktop PC I purchased is one of the XPC form factor machines by Shuttle. It is very small and looks pretty good on the desktop. It really makes no sense to have a big box that is mostly filled with air anyway. If I buy any more desktops in the future (might just stick to laptops), they will definitely be a small form factor.